Referring to FIG. 1, a circuit 100 is shown for filtering blood from a patient 105, in a well-known medical procedure referred to as hemodialysis. In hemodialysis, blood flows through an arterial channel 120 to an arterial pressure sensor 130. The arterial pressure sensor 130 includes a transducer so that the pressure of the blood flowing through the circuit 100 on the arterial side can be monitored. The blood then flows through a portion of the channel that abuts a pump 140, such as a peristaltic pump, usually a roller type tube pump as indicated. The pump 140 forces the blood through the circuit 100. In some cases, the pressure sensor 130 is after the pump 140. The blood then flows to a dialyzer 150 and then to a venous pressure sensor 160. Next, the blood flows through the entry port of a chamber 170 in which any gas in the blood, such as air, can escape. After leaving the chamber 170, the blood travels through a venous line 180 and back to patient 105.